Kettlebells

Swings
When teaching two hand swings, it can be a toss-up as to which style kettlebell to use. On one hand, the curved handle of a cast-iron kettlebell is easier to hold than the flat-handled competition kettlebell, however, since the handle's vary in size depending on the weight of the cast-iron kettlebell, the lighter weight kettlebell's have very narrow handles which are difficult to hold with two hands. Once beyond the learning phase, the curved handle of the cast-iron kettlebell is the clear winner for swings.

Goblet Squats
The curved handle of a cast-iron kettlebell is also preferred when performing Goblet Squats. Since the handle remains the same diameter all the way to the body of the kettlebell, the hand can get a firm grip on the handle, while the curve of the handle allows the wrist to stay more in line with the forearm.

Cleans
When performing double cleans, (or double swings or double snatches) an interesting thing can happen when using very light competition style weights. Since the kettlebell is hollow, the walls of the kettlebell must be thin. As a result, if the kettlebell's contact each other on the way up or down they will have a tendency to bounce off of each other like basketballs. As they get heavier this happens less and less, but it must be addressed with beginners or anyone using lighter weights. The last thing you want is for the kettlebells to bounce away from each other on the way down and hit the user on the legs. Another item to consider is that when hiking two large kettlebell's through the legs, regardless of weight, the stance used needs to be wide enough to allow room for them to pass.